A simple photonic crystal fiber (PCF) model containing six large air holes in cladding is used. By introducing a small air hole in the core center, the effective index of the core mode is lowered. Therefore the tough problem of phase matching between core and plasmon modes in the surface plasmon resonance (SPR) sensor is resolved. At the same time, the analysis method based on loss spectrum is used to research the sensing effects of the parameters variation in the PCF SPR sensor. These parameters are optimized and the best structure of PCF SPR sensor is acquired. The sensitivity and resolution are also given. The maximum spectral sensitivity of the refractive index sensor is 0.075 μm. Its resolution reaches 1.33×10-4 if the spectrometer has a demodulation resolution of 1%. This sensor has a comparable resolution with other existing sensors, but it has much simpler structure.